1. Field of the Invention
The present invention relates to a scanning optical apparatus for scanning a light outputted from a light source on a prescribed scanning surface.
2. Description of the Related Art
In image forming apparatuses such as printers, copiers, and facsimiles, a scanning optical apparatus, which scans a beam light for writing an electrostatic latent image on an image supporter, is employed for the purpose of writing said electrostatic latent image on said image supporter such as photoreceptor drums.
Such scanning optical apparatus employs a polarizer such as polygon mirrors for converging the beam light into scanning light. The beam light from a light source is converged on the surface of the polarizer, and then, converged again on the image supporter (hereinafter referred to as “photoreceptor drum”) by a lens (so called, f. theta. lens). That is, the beam light is coupled in relation to the surface of the polarizer and the photoreceptor drum, thereby correcting an optical face tangle error of the polarizer. Additionally hereinafter, a scanning direction of the beam light by the polarizer is referred to as “main scanning direction”, and a direction at right angles to the traveling directions of the main scanning direction and the beam light is referred to as “sub scanning direction”.
In recent years, as a lens for converging the beam light on the image supporter, for example, a plurality of lenses (such as, a scanning lens and a correcting lens) are employed, arranged on the light path of a single beam light as illustrated in a patent literature 1 (Unexamined Japanese Patent Publication No. H9-80331).
As indicated in Patent literature 1, the followings are benefited by using a plurality of lenses. That is, when a plurality of lenses is used, controllable parameters generally increase, thereby making it easy for the optical design to be adaptable to various conditions.
For example, spherical aberration can generally be suppressed small, when the beam light fallen on a lens has a narrower spread (in short, the beam light should be fallen on as near the center of the lens as possible). Therefore, when using a plurality of lenses, it becomes possible to employ a method for converging the beam light in a phased manner by such plurality of lenses, and thus, the spherical aberration of when the beam light is converged on the photoreceptor drum is suppressed smaller than that of when the beam light is converged by a single lens. This enables density growth of the beam light on the photoreceptor drum, thereby increasing the writing speed of the electrostatic latent image.
FIG. 1, as illustrated in Patent literature 1, shows a schematic cross-sectional view of a printer B (image forming apparatus) employing a scanning optical apparatus X2 according to a conventional example, in which a plurality of lenses is arranged in between a polarizer and a photoreceptor drum. Hereinafter, as referring to FIG. 1, Scanning optical apparatus X2 in accordance with a conventional example, as well as Printer B using the same are explained.
Printer B shown in FIG. 1 comprises a printing member α1 for forming a toner image and printing it onto printing paper, a paper feeder α2 for feeding the printing paper to Printing member α1, and a paper discharger α3 for discharging the printing paper on which printing has been conducted. Through an external input interface not shown, a prescribed printing request signal indicating a printing request, as well as a image data signal indicating image data are inputted from an external device (typically, a personal computer) connected to Printer B. The image data is read by an image processing controller not shown based on the image data signal, and then transformed into gray value data relative to each of four colors: black (BK), magenta (M), yellow (Y), and cyan (C).
Printing member α1 schematically comprises, such as; photoreceptor drums 1BK, 1M, 1Y, and 1C corresponding to each of said four colors; Scanning optical apparatus X2; developers 7BK, 7M, 7Y, and 7C corresponding to each of the colors; a intermediate transfer belt 8; various types of rollers 9a, 9b, and 9c; and a fixing apparatus 10. Said image processing controller controls four light sources 2 (see FIG. 7, black light source 2BK, magenta light source 2M, yellow light source 2Y, and cyan light source 2C) based on the gray value data for illuminating a light onto each of Photoreceptor drums 1 (black Photoreceptor drum 1BK, magenta Photoreceptor drum 1M, yellow Photoreceptor drum 1Y, and cyan Photoreceptor drum 1C) which correspond to four colors black (BK), magenta (M), yellow (Y), and cyan (C), thereby illuminating a beam.
The beam is guided to the above mentioned each of Photoreceptor drums 1 by Scanning optical apparatus X2 having such as a plurality of deflecting mirrors 3, polarizer 4, and each of lenses 5, 6 as described later in details, thereby forming an electrostatic latent image on the surface of each Photoreceptor drum 1.
Additionally, the toner on developing rollers in Developers 7 (black Developer 7BK, magenta Developer 7M, yellow Developer 7Y, and cyan Developer 7C) corresponding to each of Photoreceptor drums 1 is pulled onto the surface of each of Photoreceptor drums 1, and then, by the toner, an electrostatic latent image is developed as a toner image according to the electric potential gap (developing bias) between each of Photoreceptor drum 1 and each of the developing rollers.
Paper feeder α2 schematically comprises such as a paper cassette 11 and a paper feeding roller 12. Printing paper is previously set in Paper cassette 11. According to a printing request from a user (for instance, an operation input from an operation panel installed in the exterior of Printer B), Paper feeding roller 12 is rotary-driven by the control of the image processing controller, thereby delivering the printing paper in Paper cassette 11 into Printing member α1.
The printing paper from Paper feeder α2 is delivered by a delivering roller 9a. Also, on a registration roller 9b, the printing paper is set in the suspended state for a proper time. This enables adjustment of timing of the printing paper reaching to a nip between Intermediate transfer belt 8 and a secondary transfer roller 9c. On the other hand, the toner image formed on each of the Photoreceptor drums 1 is transferred to Intermediate transfer belt 8, and then, by the drive of the same, transferred onto the printing paper passing through the nip between Intermediate transfer belt 8 and Secondary transfer roller 9c. Then, the printing paper on which the toner image was transferred is delivered to Fixing apparatus 10, and then fixed onto the printing paper by, for example, such as a heat roller. The printing paper on which the toner image was fixed is then delivered to Paper discharger α3 and discharged.
Scanning optical apparatus X2 is for guiding each of the beam lights outputted from a plurality of Light sources 2 for writing an electrostatic latent image to each of corresponding Photoreceptor drums 1, and at the same time, for scanning said beam lights thereon.
FIG. 7 shows a general structure of Scanning optical apparatus X2. Hereinafter, as referring to FIGS. 1 and 7, Scanning optical apparatus X2 in accordance with a conventional example is explained. In addition, as mentioned above, Scanning optical apparatus X2 is applicable to tandem type Printer B, in which totally four light paths for guiding the beam light to each of four Photoreceptor drums 1 (1BK, 1M, 1Y, and 1C) are formed. However, in FIG. 7, one of four light paths is hypothetically shown for simplicity.
Scanning optical apparatus X2 includes; Light sources 2 corresponding to each of the above-mentioned four colors (Black light source 2BK, Magenta light source 2M, Yellow light source 2Y, and Cyan light source 2C); collimator lenses 13 corresponding to each of the four colors (black collimator lens 13BK, magenta collimator lens 13M, yellow collimator lens 13Y, and cyan collimator lens 13C); an aperture 14; a cylindrical lens 15; a polarizer 4; a scanning lens 5 common between the four colors; correcting lenses 6 corresponding to each of the four colors (black correcting lens 6BK, magenta correcting lens 6M, yellow correcting lens 6Y, and cyan correcting lens 6C). Scanning optical apparatus X2 also includes such as one or a plurality of deflecting mirrors corresponding to each of the four colors (black deflecting mirror 3BK1, magenta deflecting mirrors 3M1, 3M2, and 3M3, yellow deflecting mirrors 3Y1 and 3Y2, and cyan deflecting mirrors 3C1 and 3C2), however not shown in FIG. 7.
The beam light outputted from each of Light sources 2 is transformed into a parallel light (the light with no diameter changes relative to the traveling direction) by passing through Collimator lens 13. Also, the beam light is shaped by passing through Aperture 14. Furthermore, the beam light passes through Cylindrical lens 15, and by the light condensing effect thereof, converges near the surface of Polarizer 4, such as polygon mirrors or MEMS (MicroElectroMechanical system) mirrors. Polarizer 4 rotates about its rotating shaft center 4a, and thereby transforming the beam light into a scanning light.
FIG. 8 shows a cross-sectional view along a sub scanning direction of a scanning optical apparatus X1 according to an embodiment of the present invention, and more particularly, a cross-sectional view along a bisector S2-S2 of the scanning range of the beam light shown in FIG. 7.
As shown in FIG. 8, the beam light converged and reflected on or near Polarizer 4 falls on Scanning lens 5, and then is refracted by Scanning lens 5 such that, after the output, the light flux is reduced in the sub scanning direction along with the progression. Also, as being reduced, the beam light falls on each of Correcting lenses 6 corresponding to each of the colors. Correcting lens 6 is a spherical surface shape lens with its cross-sectional shape in the sub scanning direction having a fixed curvature. The beam light is converged on the surface of each Photoreceptor drum 1 by refraction of Correcting lens 6. With the beam light which converges in this way scanned on the surface of each Photoreceptor drum 1, an electrostatic latent image is written on each Photoreceptor drum 1.
In the above-mentioned structure, it is possible to suppress spherical aberration of Correcting lens 6 by gradually converging the beam light by means of Scanning lens 5 and Correcting lens 6, thereby realizing density growth of the beam light on each Photoreceptor drum 1. This enables improvement of the writing speed and the image quality of an electrostatic latent image.
However, the following problems are still concerned in the above-mentioned conventional example.
As has been well-known, sub scanning magnification β between Polarizer 4 and Photoreceptor drum 1 (the ratio between the size of the image on Polarizer 4 and the size of the image on Photoreceptor drum 1) depends on the ratio between a distance T from Polarizer 4 to the scanning position of the beam light on Photoreceptor drum 1 and a distance L3 from Polarizer 4 to the reduction starting point at which the beam light starts reducing. Particularly, in general, the smaller T is relative to L3, the larger the sub scanning magnification β increases. Additionally, in the case of FIG. 7, the distance L3 equals to the distance L2 from Polarizer 4 to Scanning lens 5.
When the sub scanning magnification β increases, the following problems occur. That is, as shown in FIG. 8, the incident position of the beam light on Polarizer 4 is displaced from the point A1 to the point B1 for the amount of ΔX. Accompanying with such displacement of the incident position of the beam light relative to Polarizer 4, displacement for the amount of ΔS may occur also in the incident position of the beam light relative to Photoreceptor drum 1. Such relationship between the displacement amounts of ΔX and ΔS can be represented in the following expression (1)ΔS=|β|ΔX  (1)
In short, the sub scanning magnification β is a magnification ratio of the displacement of the beam light, and when such sub scanning magnification β is large, the displacement of the beam light in the sub scanning direction on Photoreceptor drum 1 becomes large. This makes it difficult to keep the scanning path of the beam light to be linear on Photoreceptor drum 1 (so-called field curvature becomes large), and cannot maintain the quality of an image formed in an image forming apparatus.
In order to maintain the sub scanning magnification β small, the beam light reduction should be started as far from Polarizer 4 as possible (magnifying L3 relative to T) with Scanning lens 5 put away from Polarizer 4, however, the following problems may occur in such case.
In FIG. 9, the cross-section in the main scanning direction of Scanning optical apparatus X2 is illustrated in two ways: (a) when Scanning lens 5 is close to Polarizer 4, (b) when Scanning lens 5 is far from Polarizer 4.
As shown in FIG. 9(a), when Scanning lens 5 is close to Polarizer 4, Scanning lens 5 short in the main scanning direction can be used, since Scanning lens 5 deflects the beam light before the scanning range of the beam light scanned by Polarizer 4 is magnified in the main scanning direction. Similarly, Correcting lens 6 short in the main scanning direction can be used.
On the other hand, as shown in FIG. 9(b), when Scanning lens 5 is pulled away from Polarizer 4, Scanning lens 5 which is long enough for the scanning range of the beam light widely magnified in the main scanning direction is needed. Therefore, when Scanning lens 5 is pulled away from Polarizer 4 in order to maintain the sub scanning magnification β small, Scanning lens 5 is elongated in the main direction. Additionally, since Correcting lens 6 is generally longer than Scanning lens 5 in the main scanning direction, the elongation of Scanning lens 5 is synonymous with the elongation of Correcting lens 6, and consequently, resulting in size growth of the entire scanning optical apparatus.
As above, in the conventional example, it was impossible to strike a balance between maintaining the sub scanning magnification β small (decreasing field curvature) and downsizing the size of a scanning optical apparatus.
Consequently, this invention has been invented considering the foregoing conditions, and the purpose of this invention is to provide a scanning optical apparatus capable of maintaining the sub scanning magnification β small, and at the same time, keeping the size of the apparatus small without elongating a lens.